The dehumidification of air or a humid gas mixture by using solid desiccants is well known in the art, which includes a static drying and a dynamic drying process. The static drying process comprises contacting the humid air with the moisture absorbing desiccants such as calcium sulfate, potassium hydroxide, calcium chloride so that moisture is removed therefrom. As a result the humid air is dehumidified. This static drying process can not satisfactorily undergo for a substantially long period due to the solid desiccants being not able to be regenerated in-situ after absorbing moisture. Therefore, it is not suitable for dehumidifying a large circulation of humid air. The dynamic drying process is a continuous process, in which adsorbing of moisture and regeneration of adsorbent are carried out simultaneously and independently. Typical adsorbents used in the dynamic process, for example, are silica gel, molecular sieves, and alumina. The dynamic drying process in general can be carried out in a two-tower fluidized bed dehumidifier or in a rotary honeycomb wheel dehumidifier. The two-tower fixed bed dehumidifier mainly contains two towers, each of which is filled with absorbent particles, and valves for shifting the humid air from the first tower to the second tower and at the same time shifting the regenerated hot air from the second tower to the first tower. This type of dehumidifier has several drawbacks such as a large pressure gradient in the fixed particle bed which leads to a significant power consumption, and an oscillating variation in humidity of the dehumidified air. The rotary honeycombed wheel dehumidifier contains a slow rotating wheel constructed by a cylindrical matrix having longitudinal passages and adsorbent deposited on the surface of the matrix. The humid air and the regenerated hot air are introduced separately and simultaneously to two different portions of the rotating wheel such that the adsorbent is periodically desorbed during the continuous dehumidification process. The rotary honeycombed wheel dehumidifier has the following advantages: a lower pressure gradient across the wheel, an increased absorbing surface area per unit volume, and a quick and effective absorbing/desorbing ability. As a result, the power consumption is lowered, the dehumidification efficiency is enhanced, and the operation is easier when a rotary honeycombed wheel dehumidifier is used in comparison with a two-tower fixed bed dehumidifier.
A desired moisture exchange honeycombed wheel to be used in the above-described rotary dehumidifier should be small in heat capacity and light weighted, and should have excellent mechanical properties, and high moisture adsorbing capacity.
Japanese Invention Kokai 55(1980)-142522 discloses a process for manufacturing a moisture exchange element which comprises impregnating a porous sheet such as paper with moisture adsorbent solution such as an aqueous solution of calcium chloride, drying the impregnated sheet, adhering the dried sheet with a corrugated blank sheet to form a laminated sheet, cutting the laminated sheet into desired length, and laminating the cut sheets one above the other with adhesives to form a cubic moisture exchange element. In this process, the corrugated blank sheet is used to increase the mechanical strength of the element, and no adsorbent is deposited on its surface.
Japanese Invention Kokai 60(1985)-175521 discloses a process for manufacturing a dehumidifying element with a number of small through holes, in which an element with a number of small through holes is formed from paper made of organic fiber and inorganic fiber, the formed element is heated under a supply of insufficient volume of oxygen to carbonize the organic components contained in the element, and then is impregnated with a dehumidifying agent, wherein the formed element is impregnated with inorganic reinforcing agent before or after the carbonization.
Japanese Invention Kokai 63(1988)-175619 discloses a process for preparing a moisture absorbing strip, which comprises soaking a paper strip made from pulp and inorganic fiber, e.g. ceramic fiber, in water glass, semi-drying the soaked paper, passing the semi-dried paper through a pair of special rollers to form a corrugated strip, soaking the corrugated strip in an acid to produce silica hydrogel, and then washing with water to remove the salts formed as side products, and finally heating the resulting silica hydrogel deposited corrugated strip to remove the water contained therein by vaporization. A moisture-exchange honeycombed wheel can be formed on a roller by coiling up the corrugated strip along with a silica hydrogel deposited flat strip, wherein an adhesive is applied on the peaks of the corrugated strip immediately before the coiling for adhering the flat strip to the corrugated strip.
Several problems were confronted in carrying out the Japanese Kokai '619 process and we found that the problems are mainly caused by the water glass used. For example, the gelling rate of water glass is considerably too fast due to a high content of sodium ions. Consequently, the impregnating life of water glass bath is short, and the properties of the impregnated paper will vary after the water glass bath being used for a period of time. This fast gelling rate also affects the average pore size of the silica hydrogel adsorbent formed thereafter, which is about 20 .ANG.. A moisture adsorbent having this level of average pore size does not have a satisfactory adsorbing capacity, which causes the adsorbent to be desorbed more frequently and is not suitable for dehumidifying a high-humidity air. Moreover, this fast gelling rate also makes the impregnated paper strip difficult to be semi-dried to an adequate degree to form a corrugated strip. In addition, the pH value of water glass is about 11.5, therefore, large quantities of acid are required in the acid catalyzed gelling reaction. Furthermore, the sodium salts formed as a side product in the acid catalyzed gelling reaction will deposite on the paper substrate, which not only increases the weight of moisture adsorbing strip, but blocks the pores of the adsorbent. These render the washing step inevitable. Yet, the sodium salts may still remain on the paper substrate after the washing. The residue of sodium salts will be gradually carried away in the adsorbing/desorbing operation and adversely affect the life of the dehumidifier. In addition, the silica hydrogel formed on the paper substrate may also be washed away with the sodium salts during the washing operation. More importantly, washing with water is time and labor consuming, and requires a large supply of water and complicated waste water treatments.
The objects of the present invention are to provide a process for preparing a moisture exchange element eliminating the drawbacks mentioned above; employing less number of steps than the prior art process and simplifying the execution; and increasing the amount of adsorbent deposited on the substrate and the adsorbing capacity of adsorbent deposited.